The invention relates generally to a color electronic reprographic printing system, and more particularly concerns a method and apparatus for distinguishing between opaque and transparent sheets to which are applied a plurality of developed images in a color reprographic system and for reliably sensing the presence of a transparent sheet in such a system in which the sheet's movement is not closely controlled.
The marking engine of an electronic reprographic printing system is frequently an electrophotographic printing machine. In an electrophotographic printing machine, a photoconductive member is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is thereafter selectively exposed. Exposure of the charged photoconductive member dissipates the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational area contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image on the photoconductive member which is subsequently transferred to a copy sheet. The copy sheet is heated to permanently affix the toner image thereto in image configuration.
Multi-color electrophotographic printing is substantially identical to the foregoing process of black and white printing. However, rather than forming a single latent image on the photoconductive surface, successive latent images corresponding to different colors are recorded thereon. Each single color electrostatic latent image is developed with toner of a color complementary thereto. This process is repeated a plurality of cycles for differently colored images and their respective complementarily colored toner. Each single color toner image is transferred to the copy sheet in superimposed registration with the prior toner image. This creates a multi-layered toner image on the copy sheet. Thereafter, the multi-layered toner image is permanently affixed to the copy sheet creating a color copy. The developer material may be a liquid or a powder material.
In the process of black and white printing, the copy sheet is advanced from an input tray to a path internal the electrophotographic printing machine where a toner image is transferred thereto and then to an output catch tray for subsequent removal therefrom by the machine operator. In the process of multi-color printing, the copy sheet moves from an input tray through a recirculating path internal to the printing machine where a plurality of toner images is transferred thereto and then to an output catch tray for subsequent removal. With regard to multi-color printing, a sheet gripper secured to a transport receives the copy sheet and transports it in a recirculating path enabling the plurality of different color images to be transferred thereto. The sheet gripper grips one edge of the copy sheet and moves the sheet in a recirculating path so that accurate multi-pass color registration is achieved. In this way, magenta, cyan, yellow, and black toner images are transferred to the copy sheet in registration with one another.
In a color reprographic system, a transparent, polymer copy sheet is developed and fused to different parameters than an opaque, paper copy sheet. It is therefore desirable to distinguish between opaque and transparent copy sheets. As is well known in the art, previous systems have used two-piece transmissive sensors to distinguish copy sheets. Such sensors have an emitter and a photodetector disposed on opposite sides of the path along which the copy sheet moves. An opaque copy sheet interrupts the light transmitted from the emitter to the photodetector while a transparent copy sheet does not. Although this is an effective technique for distinguishing transparent from opaque copy sheets, such sensors are costly, relatively large, cumbersome to locate within the system, and special-purpose. Such sensors are used, for example, in the Canon CLC-1 and CLC-500 color photocopiers.
Another type of sensor, which is commonly used for copy sheet sensing in reprographic systems, is a diffuse reflective sensor. In this sensor, the emitter and photodetector are disposed on the same side of the copy sheet path. A matte black background or open space is disposed on the other side. When no copy sheet is in front of the sensor, light from the emitter is absorbed by the background or open space. When a copy sheet is placed in front of the sensor, light from the emitter is reflected off of the copy sheet and transmitted to the photodetector. In known configurations, such a sensor detects both transparent and opaque copy sheets--it does not distinguish between them. Such a sensor is used, for example, in the Model No. 5046 photocopier manufactured by the Xerox Corporation.
Simply detecting the presence of a transparent copy sheet presents significant difficulties. Mechanical switches placed in the copy sheet path can detect either a transparent or opaque copy sheet. However, such switches are unreliable. As noted above, diffuse reflective sensors can be used to detect both transparent and opaque copy sheets. Opaque copy sheets are readily detected because the reflected light is scattered, or diffused, from the surface of the copy sheet, so that a detectable amount of light will reach the detector over a wide range of angles of the plane of the sheet relative to the axis of the sensor. However, transparent copy sheets produce specular, rather than diffuse, reflection of incident light from the sensor's emitter. Thus, a detectable amount of light will reach the detector only over a narrow range of angles of the sheet plane relative to the sensor axis. This limitation is acceptable in applications in which the copy sheet position is closely controlled. However, this limitation is not acceptable where, as in portions of the copy sheet circulation path of the color reprographic system described herein, the leading and trailing portions of the copy sheet are closely controlled while the body portion is not. In such applications, the known diffuse reflective sensor configuration will not reliably detect transparent copy sheets.